1. Field of the Invention
The present invention relates generally to a method and apparatus for retrieving subterranean core samples under pressure and, more specifically to a method and apparatus for recovering core samples under insitu pressure and temperature.
2. Background of the Invention
The recovery of subterranean, geologic samples is commonly performed by an operation or technique referred to as coring. This technique has evolved from simple single tube systems to dual tube systems that are most commonly used in the mining and petroleum industry today. Because such coring techniques are employed for recovery of volatile components contained within rock samples, various modifications have been made to conventional coring devices in order, for example, to retain formation pressure on the core during recovery.
In order to accurately analyze the composition of certain volatile core samples, the core sample must maintain its chemical, mechanical, and/or physical integrity during the retrieval process. Downhole, water or other substances in the formation may contain dissolved gases which are maintained in solution by the extreme pressure exerted on the fluids when they are in the formation. Thus, unless a pressure core barrel is employed during the core extraction process, the pressure on the core at the surface will differ dramatically from the pressure experienced on the core sample downhole. Furthermore, as the pressure on the core sample decreases, fluids in the core will expand and any gas dissolved therein will come out of solution. Accordingly, the retrieved core sample will not accurately represent the composition of the downhole formation.
One common method of retaining core integrity is known as pressure coring. Pressure coring utilizes various apparatuses to maintain the core sample at or near formation pressure as the core is retrieved to the surface. Core sampling tools that include pressurized core barrels have been known for several decades. For example, U.S. Pat. No. 2,248,910 to D. W. Auld et al. entitled xe2x80x9cPRESSURE RETAINING CORE BARRELxe2x80x9d discloses a core barrel that is sealed downhole to maintain the core at downhole pressure. U.S. Pat. No. 3,548,958 to Blackwall et al. discloses another pressure core barrel that utilizes a compressed gas system to maintain pressure on the core sample during the core retrieval process. U.S. Pat. No. 4,317,490 to Milberger et al. discloses yet another pressurized core barrel in which a ball valve, actuated from the surface is employed to trap ambient pressure in the core barrel while downhole. U.S. Pat. No. 4,466,495 to Jageler discloses a pressure core barrel of a sidewall coring tool. Other pressure core barrels are disclosed in U.S. Pat. No. 4,356,872 to Hyland, U.S. Pat. No. 4,256,192 to Aumann, the inventor of the present invention, U.S. Pat. No. 4,230,192 to Pfannkuche, U.S. Pat. No. 4,142,594 to Thompson et al., U.S. Pat. No. 4,014,393 to Hensel, Jr., and U.S. Pat. No. 4,735,269 to Park et al. Pressure core barrels often utilize pressure actuation to release a latch and/or mechanical manipulation of the drill pipe to close a valve and also often require the entire core barrel to be brought to the surface to recover the core.
Encapsulation is another technique known in the art to maintain the integrity of unconsolidated or friable core samples. In U.S. Pat. No. 4,449,594 to Sparks, a foam is introduced into the well under a correlated control pressure. The core sample is thus encapsulated while the reservoir pressure within the sample is balanced by the bottom hole foam balance pressure to produce a balanced, pressurized core sample. Another method of encapsulating a core sample is disclosed in U.S. Pat. No. 4,716,974 to Radford et al. in which a liquid foam is allowed to cure to form a sponge-like solid that retains oil as the core is depressurized during retrieval. Another attempt to stabilize cores where unconsolidated and friable columnar masses of earth can be handled without altering the characteristics of its physical structure employs a rubber sleeve that encapsulates the core sample. A housing is provided for positioning the ensleeved core therein and subfreezing material is circulated around the ensleeved core to freeze and solidify the core fluids contained therein. Likewise, in U.S. Pat. Nos. 5,360,074, 5,560,438, 5,546,798, and 5,482,123 to Collee et al., methods for maintaining the mechanical integrity and for maximizing the chemical integrity of a core sample during transport from a subterranean formation to the surface comprises employing an encapsulating material that increases in viscosity or even solidifies at temperatures slightly lower than those expected downhole. The patents to Collee note that in such a method of encapsulation, the chemical integrity of the core sample can be further increased by using a pressure core barrel.
Certain core samples, however, such as cores containing methane hydrate, not only require that the core sample be maintained at formation pressure when brought to the surface for examination and testing, but because methane hydrate is a material stable only within a limited pressure/temperature range, the core sample must also be maintained at formation temperature during recovery. If the core sample is allowed to become heated above this pressure/temperature envelope during the extraction process, the structural and physical makeup of the sample will be partially if not totally lost.
One attempt in the art to retrieve methane hydrate cores is disclosed in U.S. Pat. No. 4,371,045 to McGuire et al. As described, the cores are cooled down to at least xe2x88x9280 degrees C at which temperature the pressure of methane hydrates is 1 atmosphere. Such cooling is accomplished by employing a conventional wire line retrievable core barrel having perforations therein through which cryogenic liquid passes into direct contact with the hydrocarbon hydrates and thus thermodynamically stabilizes the core. The invention employs an insulated chilling vessel into which the perforated core barrel and thus the core sample is moved for cryogenic freezing.
Many of the aforementioned coring apparatuses employ valves or other sealing devices to isolate the core. For example, a common method of preventing fluid access to the inner tube of a core barrel assembly is provided in U.S. Pat. No. 5,230,390 to Zastresek et al. in which a closure mechanism is configured to move from an open condition to a closed condition in response to increased fluid flow rates and pressure differentials occurring at the closure mechanism. Likewise, U.S. Pat. No. 5,253,720 to Radford et al. discloses a coring device in which a ball valve is actuated to seal off the core barrel before the core barrel is pulled to the surface.
It is also noted, that wire line retrieval of core barrels and/or manipulation of various components of the coring apparatus has previously been employed in many of these systems. For example, in U.S. Pat. No. 3,627,067 to Martinsen, a core-drilling system is disclosed in which selective or controlled release of an overshot from the core barrel while downhole is performed by pumping a wire line to which the overshot is attached up and down a prescribed number of times. In U.S. Pat. No. 3,667,558 to Lambot, an upward pull on a cable unlatches the coring head and also vents water under pressure so that it no longer forces the assembly downward. Continued pulling on the cable retrieves the coring head and the core sample. U.S. Pat. No. 3,739,865 to Wolda, discloses a wire line core barrel system that includes flexible latch fingers and provides a predetermined pressure signal indicating latching and further blocks fluid flow until the core barrel is properly latched. U.S. Pat. No. 4,800,969 discloses yet another wire line core barrel assembly in which an inner tube assembly can move down faster than the fluid flow in the drill stem during the time the inner tube assembly moves downwardly in the drill stem. U.S. Pat. No. 4,466,497 to Soinski et al. discloses yet another wire line core barrel apparatus.
Other coring systems and devices are known such as the coring apparatus disclosed in U.S. Pat. No. 3,874,465 to Young et al. in which core samples of relatively soft formations may be retrieved. The coring apparatus comprises a core barrel with an interior surface having properties similar to synthetic rubber, two semi-tubular rigid portions joined along the adjacent edges by a flexible material, and a core catcher having a plurality of flexible segments adapted to open while the core is being drilled and to close when the core is to be recovered. A latch for retaining the tool in position within the coring bit and a swivel allowing the core barrel and catcher to remain stationary while the coring bit is rotated are also provided.
While the aforementioned references disclose various methods and apparatuses for retrieving core samples of subterranean formations, these methods are inadequate to maintain a core sample at least partially comprised of methane hydrate at its downhole state. U.S. Pat. No. 4,371,045, which is specifically directed to the problem of stabilizing hydrocarbon cores, requires that the core be quickly brought to the surface before cryogenic freezing of the core is performed. Thus, it would be advantageous to provide a method and apparatus for retrieving core samples that are or become unstable when removed from the downhole environment. Such a coring method and apparatus may be applicable to not only obtaining core samples of formations containing hydrocarbons, but may have utility in other coring applications where the core samples may be unconsolidated, friable, or comprised of frozen material that would otherwise not maintain their chemical or mechanical properties once exposed to ambient pressures and temperatures. In addition, the methods and apparatuses disclosed herein may have applicability to other coring devices regardless of the type of formation from which the core sample is being taken.
Accordingly, it is an object of the present invention to provide a method and apparatus for retrieving geological core samples in which the core samples are recovered at in situ pressure.
It is another object of the present invention to provide a method and apparatus for retrieving geological core samples in which the integrity of the core sample is maintained by cooling the core sample as it is brought to the surface.
It is an object of the present invention to provide a method and apparatus for retrieving geological core samples in which heat is diverted away from the core.
It is yet another object of the present invention to provide a method and apparatus for retrieving geological core samples in which the core sample can be safely extracted into a transfer, storage, or other laboratory container while maintaining in situ pressure on the core.
It is still another object of the present invention to provide a method and apparatus for retrieving geological core samples in which the system is easily repairable.
Another object of the present invention is to provide a method and apparatus for retrieving geological core samples in a nearly continuous coring operation in which downtime is significantly reduced.
Still another object of the present invention is to provide a method and apparatus for retrieving geological core samples in which the system is reliable and relatively easy to test, maintain, and operate.
Yet another object of the present invention is to provide a method and apparatus for retrieving geological core samples in which the system is capable of various modes of operation depending on the needs of the operator.
Additional objects and advantages of the present invention will be apparent from the description and claims which follow or may be learned by practicing the invention.
Accordingly, the foregoing objects and advantages are realized in an improved method for coring and coring tools for recovering core samples under pressure comprising an inner barrel having a first end and a second end. A remotely actuable valve is connected to the inner barrel at the second end and a removable plug is attached to the first end of the inner barrel. The inner barrel, the valve, and the plug define a pressure or core sample chamber.
The coring tool further includes a cooling system associated with the inner barrel for cooling the inner barrel during retrieval of the core sample to the surface. Preferably, the cooling system comprises a plurality of thermal electric coolers which cool an inner tube of the inner barrel. The thermal electric coolers are thus disposed along a portion of the inner tube.
In another preferred embodiment, the cooling system comprises a plurality of heat pipes extending around and along the inner tube of the inner barrel. The heat pipes may be contoured to match the shape of the inner tube for maximum efficiency in extracting heat from the inner tube.
The cooling system may also include a power source for providing electric current to a plurality of cooling elements and for providing power to a pump employed to circulate a coolant through the heat pipes.
The coring tool further preferably includes a core catcher associated with the inner barrel at an end thereof for holding a core sample within the inner barrel as the inner barrel is lifted relative to the borehole bottom. The core catcher may be comprised of a dog catcher, a basket catcher, or other types of core catchers known in the art.
The coring tool is further preferably provided with a pressure system for maintaining the pressure of the core sample at or near in situ pressure during the recovery operation when the core sample is brought to the surface. In a preferred embodiment, the pressure system comprises a piston disposed and slidable within an elongate chamber. The elongate chamber is in fluid communication with the core sample chamber at the end of the elongate chamber nearest the core sample chamber.
Preferably, the coring tool includes an outer barrel disposed about an inner barrel and further includes a coring bit secured to a distal end of the outer barrel. A sub is provided which secures the outer barrel to the inner barrel. The inner barrel comprises an outer tube and an inner tube. A swivel mechanism is preferably interposed between the outer tube and the inner tube to allow the outer tube to rotate with the rotation of the outer barrel and drill bit during drilling operations while the inner barrel system remains relatively stationary.
In a preferred embodiment, the inner barrel system comprises the core catcher, the core sample or pressure chamber, the pressure control system, and the temperature control system. The inner tube is selectively longitudinally movable relative to the outer tube for lifting the core and closing the valve. Preferably, the valve is a ball valve comprising a ball housing, a ball having a bore extending therethrough and pivotally disposed within the ball housing, and a linkage mechanism interconnected between the ball and the outer tube for closing the ball when the outer tube moves longitudinally relative to the inner tube. A catch mechanism is also provided for engaging a ball valve operator when the inner tube assembly is longitudinally moved relative to the outer tube assembly. The catch mechanism is preferably spaced a sufficient distance from an engageable point of the ball valve operator to allow a distal end of a core sample to pass completely through the ball valve before the ball valve is closed.
This relative longitudinal movement is preferably accomplished by employing selectively releasable latching mechanisms for selectively securing the inner tube system to the outer tube system. In addition, the inner barrel is longitudinally movable relative to the outer barrel for recovering the inner barrel while leaving the outer barrel downhole. This relative longitudinal movement is also preferably accomplished by employing a second selectively releasable latching mechanism for selectively securing at least a portion of the inner barrel to the outer barrel.
In order to keep the core sample adequately cool during extraction, the coring tool in accordance with the present invention preferably comprises an inner tube having a layer of insulation disposed substantially around the inner tube and an outer shell disposed substantially around the layer of insulation. The cooling system is associated with the inner tube for cooling the inner tube and thus removing heat therefrom. Because heat may be conducted away from the inner tube the inner tube is preferably comprised of a metal material. In addition, the layer of insulation may be comprised of a foam material or an evacuated annular chamber. In order to strengthen the inner tube so that it is less susceptible to downhole hydrostatic pressures, the outer shell may be comprised of steel and/or a layer of glass or carbon fiber and epoxy. A second layer of carbon fiber and epoxy may also be disposed over the inner tube.
Preferably, the coring system in accordance with the present invention includes a wireline latching system for operating the coring tool. As such, a first latching mechanism interposed between the outer barrel and the inner barrel may, by wireline, selectively latch the outer barrel to the inner barrel. Moreover, a second latching mechanism interposed between the outer tube and the inner tube may be employed for selectively latching the outer tube to the inner tube. A wireline pulling tool configured to be selectively engageable with a proximal end of the inner barrel is configured to disengage the second latching mechanism and longitudinally move the inner tube relative to the outer tube. The wireline pulling tool is also configured to disengage the first latching mechanism and retrieve the inner barrel relative to said outer barrel. A second wireline pulling tool is configured to be selectively engageable with a proximal end of the inner barrel and to leave the second latching mechanism in an engaged position locking the inner tube relative to the outer tube and to disengage the first latching mechanism and retrieve the inner barrel relative to the outer barrel.
In operation, geological core samples are retrieved by drilling a core sample, lifting the core sample into a chamber, sealing the chamber around the core sample, retrieving the chamber and core sample contained therein while leaving an associated outer barrel and drill bit downhole, and cooling the chamber as the chamber and core sample contained therein are brought to the surface. Drilling is preferably accomplished by rotating the outer barrel assembly and a drill bit attached thereto into a subterranean formation while allowing the inner barrel assembly to remain substantially rotationally stationary relative to the formation. When drilling is complete, the chamber is unlatched from the inner barrel assembly and the chamber is lifted relative to the inner barrel assembly until the core sample is contained within the chamber. The core sample is then sealed within the chamber by closing a pressure tight valve to seal the core sample within the chamber. The core sample is then recovered by unlatching the inner barrel assembly from the outer barrel assembly and raising the inner barrel assembly to the surface. Preferably, these operations are accomplished by employing a wireline tool.
Once the chamber containing the core sample has been brought to the surface, a transport container is attached to the core chamber and the core sample is transferred from the core chamber to the transport container. Preferably, this transferring process is performed while maintaining the core sample under pressure.
In a preferred embodiment, the transport container has a distal end configured to mate with a proximal end of the pressurized core retrieval chamber and an actuable sealing device, such as a ball valve, associated with a distal end of the transport container for selectively forming a substantially pressure tight chamber within the transport container. A transferring device, such as a hydraulic telescoping piston arrangement, is also provided having a proximal end configured to mate with a distal end of the pressurized core retrieval chamber. The transferring device includes an extendable member for extending through the pressurized core chamber to force a core sample therein into the transport container. Preferably, transport container has an internal diameter substantially the same as an inside diameter of the core chamber. The transport container also preferably includes means for regulating the pressure within said transport container, such as an external or internal pressure source.
In operation, the core sample is transferred from a core retrieval chamber under in situ pressure by attaching a transport container to a first end of the core retrieval chamber, attaching a transferring device to a second end of the core retrieval chamber, opening the first end of the core retrieval chamber, opening the second end of the core retrieval chamber, forcing the core sample from the core retrieval chamber into the transport container with the transferring device, and sealing the transport container around the core sample. In a preferred embodiment, opening the first end comprises releasing a sealing plug from the core retrieval chamber. Thus, the plug is configured to be removable relative to the inner tube assembly such that a core sample contained within the inner tube assembly is removable through the proximal end of the inner tube assembly. In addition, it is preferable that the system be configured to allow these operations to be performed by external manipulation of the apparatus.